1. Field of the Invention
This invention relates generally to the thermal cracking of hydrocarbons and more particularly concerns the thermal cracking of hydrocarbons under elevated pressure.
2. Description of the Prior Art
The thermal cracking or pyrolysis of paraffinic and naphthenic hydrocarbons is often effected in a thermal reactor, typically comprising tubular reactor coils installed in externally fired heaters. The properties of the feedstock and the conditions at which the reactor is operated determine reactor effluent product distribution. The primary reactions produce olefins essentially via a free radical mechanism. Secondary reactions involving the primary-produced olefins become important when the feedstock reaches high levels of conversion. The feedstock and conditions at which the reactor is operated are generally selected to maximize the yields of light olefins, especially ethylene. High selectivity toward the production of the light olefins and diolefins--that is, ethylene, propylene and butadiene--with minimum methane production and minimum coking in the coils leading to longer heater runs is achieved by operating the reactor heaters at high temperatures (750.degree.-900.degree. C.), short residence times (0.1-0.6 second) and low hydrocarbon partial pressures. Steam is added to the feedstock to reduce the hydrocarbon partial pressure and the amount of carbon being deposited on the reactor walls.
It is desirable to reduce fuel consumption in the thermal cracking of paraffinic and naphthenic hydrocarbons. Furthermore, depending on the feed-stock available and the demands of the marketplace, it is often desirable to shift the reactor effluent product distribution in order to increase the yield of materials that are generally regarded as by-products in the production of the light olefins and diolefins by the thermal cracking of paraffinic and naphthenic hydrocarbons. Generally the formation of by-products in the thermal cracking of paraffins and naphthenes would be favored by increasing the residence time of the feedstock and primarily-produced olefins in the reactor. Decreases in the feed rate and/or in the steam-to-hydrocarbon weight ratio and/or an increase in the reactor length would afford increased residence times. These measures are counter-productive for ethylene yield optimization as practical in state-of-the-art commercial furnaces. For example, a decrease in the feed rate would also produce an undesirable decrease in the reactor throughput. Furthermore, a decrease in the steam-to-hydrocarbon weight ratio can lead to an excessive buildup of coke on the reactor walls. Similarly an increase in reactor length can also lead to an excessive buildup of coke on the reactor walls. Heretofore, no commercially acceptable methods for increasing the yield of higher molecular weight by-products when desired from the thermal cracking of paraffins and naphthenes have been reported.